Wiring Board for Light-Emitting Element

ABSTRACT

A wiring board for light-emitting element, comprising a ceramic insulating substrate, and a conductor layer formed on the surface or in the inside of the insulating substrate, and having a mounting region mounting a light-emitting element on one surface of the insulating substrate; wherein the insulating substrate is provided with a heat-conducting pole-like conductor having a thermal conductivity higher than that of said insulating substrate; and the heat-conducting pole-like conductor is extending through the insulating substrate in the direction of thickness thereof from the light-emitting element mounting region of the insulating substrate, and is formed by the co-firing with the insulating substrate. The wiring board is produced inexpensively by co-firing, features excellent heat-radiating performance, is capable of quickly radiating the heat from the light-emitting element when the light-emitting element is mounted, and effectively prevents a decrease in the brightness of the light-emitting element caused by the heat.

TECHNICAL FIELD

The present invention relates to a wiring board for mounting alight-emitting element such as a light-emitting diode (LED).

BACKGROUND ART

So far, a light-emitting device obtained by mounting an LED on a wiringboard features a very high light-emitting efficiency, emits lightproducing heat in amounts smaller than that of incandescent lamps, andhas been used for a variety of applications. However, the light-emittingdevice of this kind emits light in amounts smaller than those ofincandescent bulbs and fluorescent lamps, and is used not forillumination purposes but as a source of light of an indicator consuminga current which is as very small as about 30 mA. The light-emittingdevice of this kind consumes a small electric power generating a smallamount of heat, and has, hence, been realized chiefly in a structure ofa so-called bullet type by burying the light-emitting element (LED) in aplastic insulating substrate (see patent document 1).

Accompanying the development of a light-emitting element featuring ahigh brightness and white light in recent years, further, thelight-emitting device has been much used as a back light for cell phonesand large liquid crystal TVs. However, an increase in the brightness ofthe light-emitting element is accompanied by an increase in the heatgenerated by the light-emitting device. To prevent a decrease in thebrightness of the light-emitting element, therefore, it is becomingnecessary to provide a wiring board for light-emitting element capableof quickly and highly radiating the heat produced by the light-emittingelement (see patent documents 2 and 3).

Patent document 1: JP-A-2002-124790

Patent document 2: JP-A-11-112025

Patent document 3: JP-A-2003-347600

DISCLOSURE OF THE INVENTION

The wiring board used for a light-emitting element includes a conductorlayer on the surface or in the inside of a flat insulating substrate,and has such a structure that the light-emitting element is mounted onone surface of the insulating substrate. The insulating substrate usedfor the wiring board is in many cases made of alumina. The insulatingboard made of alumina has a coefficient of thermal expansion of about7.0×10⁻⁶/° C. and a difference in the coefficient of thermal expansionfrom the printed board does not cause any practical problem. Therefore,excellent connection reliability is obtained between the two. However,the alumina has a thermal conductivity of as low as about 15 W/m·K. Tosubstitute for the alumina, therefore, attention has now been given tothe aluminum nitride having a high thermal conductivity. However, thealuminum nitride is accompanied by such defects that the startingmaterial thereof is expensive, that the firing must be effected at ahigh temperature since it can be sintered difficultly and that theprocess cost is high. Besides, due to its coefficient of thermalexpansion which is as small as 4 to 5×10⁻⁶/° C., the wiring board usingthe aluminum nitride insulating substrate arouses a problem ofdeteriorating the connection reliability when it is mounted on ageneral-purpose printed board (having a coefficient of thermal expansionof not smaller than 10×10⁻⁶/° C.) due to a difference in the thermalexpansion.

On the other hand, an insulating substrate made of a resin has acoefficient of thermal expansion close to that of the printed board andposes no problem concerning the reliability of mounting the printedboard, but has a thermal conductivity which is as very low as 0.05W/m·K, and is not quite capable of coping with the problem related tothe heat. When used in the near ultraviolet-ray band for extendedperiods of time, therefore, the insulating board is blackened causing adecrease in the brightness of the light-emitting element.

Thus, there has not yet been provided a wiring board for light-emittingelement, which is inexpensive exhibiting excellent thermal conductivityand excellent mounting reliability.

It is, therefore, an object of the present invention to provide a wiringboard for light-emitting element, which is inexpensive featuringexcellent heat-radiating performance and reliable mounting.

Another object of the present invention is to provide a light-emittingdevice mounting a light-emitting element on the wiring board.

According to the present invention, there is provided a wiring board forlight-emitting element, comprising a ceramic insulating substrate, and aconductor layer formed on the surface or in the inside of the insulatingsubstrate, and having a mounting region mounting a light-emittingelement on one surface of the insulating substrate; wherein

the insulating substrate is provided with a heat-conducting pole-likeconductor having a thermal conductivity higher than that of theinsulating substrate; and

the heat-conducting pole-like conductor is extending through theinsulating substrate in the direction of thickness thereof from thelight-emitting element mounting region of the insulating substrate, andis formed by co-firing with the insulating substrate.

In the wiring board for light-emitting element of the present invention,the heat-conducting pole-like conductor having a thermal conductivityhigher than that of the insulating substrate is extending through theinsulating substrate enabling the heat generated by the light-emittingelement to be quickly radiated out of the wiring board. Therefore, thelight-emitting element is effectively suppressed from being excessivelyheated preventing a decrease in the brightness of the light-emittingelement or enhancing the brightness of the light-emitting element.

Besides, the insulating substrate made of ceramics has a higher thermalconductivity than that of a substrate made by molding a resin, andfeatures excellent heat-radiating performance without exhibiting achange in the molecular structure that is caused by the heat generatedfrom the source of light or by the light emitted from the source oflight even after the passage of an extended period of time, withoutalmost causing a change in the color tone (blackening, etc.) ordeterioration in the characteristics, and maintaining high reliability.

In the present invention, the insulating substrate can be formed byfiring at any temperature. Here, use of a high temperature-firinginsulating substrate having a firing temperature of higher than 1050° C.is advantageous from the standpoint of enhancing the thermalconductivity while use of a low temperature-firing insulating substratehaving a firing temperature of not higher than 1050° C. is advantageousfrom such a standpoint that the wiring layer of a low-resistanceconductor such as of gold, silver or copper can be formed by theco-firing.

It is desired that the surface of the insulating substrate on the sideof the light-emitting element mounting region has a total reflectionfactor of not lower than 70% to prevent light of the light-emittingelement from transmitting through the insulating substrate or from beingabsorbed by the insulating substrate, and to enhance the light-emittingefficiency.

The heat-conducting pole-like conductor is extending through theinsulating substrate from the region on where the light-emitting elementis mounted to quickly radiate the heat generated by the light-emittingelement. It is desired that the heat-conducting pole-like conductor hasa plane sectional area greater than the mounting area (corresponds tothe bottom surface of the light-emitting element) on where thelight-emitting element is mounted. This increases the heat-radiatingportion enabling the heat generated by the light-emitting element to bemore quickly radiated.

It is further desired that the boundary portion between the end surfaceof the heat-conducting pole-like conductor and the surface of theinsulating substrate, and the vicinity thereof, are covered with aboundary protection layer formed by at least one of those selected fromthe group consisting of a metal, ceramics and a resin. Provision of theboundary protection layer works to relax the difference in the thermalexpansion between the pole-like conductor and the insulating substrateand to suppress the occurrence of cracks on the boundary.

It is further desired that the end surface of the heat-conductingpole-like conductor on the side of the mounting region (end surface ofthe upper side) and the peripheral edges thereof are covered with acovering layer containing a metal or a resin and that the end surface ofthe heat-conducting pole-like conductor on the side opposite to the sideof the mounting region (end surface of the lower side) and theperipheral edges thereof are covered with a covering layer containing atleast one of those selected from the group consisting of a metal,ceramics and a resin. Provision of the covering layer on the end surfaceon the upper side of the heat-conducting pole-like conductor or on theend surface on the lower side thereof, too, makes it possible to relaxthe difference in the thermal expansion between the pole-like conductorand the insulating substrate and to suppress the occurrence of cracks onthe boundary between the end surface of the pole-like conductor and theinsulating substrate.

It is further desired that the heat-conducting pole-like conductor has athermal conductivity of not smaller than 80 W/m·K from the standpoint ofquickly radiating the heat generated by the light-emitting element.

Further, it is desired that the heat-conducting pole-like conductor isformed by using a metal material and a ceramic material. Use of themetal/ceramic pole-like conductor makes it easy to control, for example,the coefficient of thermal expansion. Upon bringing the coefficient ofthermal expansion close to that of the insulating substrate, it is madepossible to suppress the occurrence of cracks caused by the mismatchingin the thermal expansion from the from that of the insulating substrate.It is further allowed to increase the adhering strength between thepole-like conductor and the insulating substrate, which is advantageouseven from the standpoint of conducting the co-firing with the insulatingsubstrate.

In the present invention, the heat-conducting pole-like conductor can beincorporated in a portion of the electric circuit. In this case, noconduction terminal is necessary offering advantage from the standpointof decreasing the size of the wiring board for light-emitting element.

The heat-conducting pole-like conductor may be formed by using aplurality of layers having different coefficients of thermal expansionor different thermal conductivities. When the heat-conducting pole-likeconductor is formed relying upon the above laminated-layer structure,the coefficients of thermal expansion and the thermal conductivities ofthe layers are varied, e.g., a difference in the thermal expansion isdecreased between the layer on the side of the mounting region and thelight-emitting element that is mounted. In this case, the layer of thelower side is set to possess a high thermal conductivity irrespective ofthe coefficient of thermal expansion to enhance the heat-radiatingperformance yet maintaining reliability.

In the light-emitting device mounting the light-emitting element on thewiring board for light-emitting element of the present invention asdescribed above, heat generated by the light-emitting element can bequickly radiated out of the device, suppressing a decrease in thebrightness caused by heat generated by the light-emitting element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and 1(b) are sectional views illustrating a representativestructure of a wiring board for light-emitting element according to thepresent invention;

FIG. 2 is a sectional view illustrating another wiring board forlight-emitting element according to the present invention;

FIGS. 3(a) and 3(b) are sectional views illustrating a further wiringboard for light-emitting element according to the present invention;

FIG. 4 is a view illustrating preferred examples of the shape of theside surface of a heat-conducting pole-like conductor formed on thewiring board for light-emitting element according to the presentinvention;

FIG. 5 is a view illustrating a conductor layer that is provided whenthere is formed a step in the side surface of the heat-conductingpole-like conductor;

FIG. 6 is a view illustrating a method of incorporating a pole-likeconductor in an insulating substrate in the wiring board forlight-emitting element according to the present invention;

FIG. 7 is a view illustrating another method of incorporating apole-like conductor in an insulating substrate in the wiring board forlight-emitting element according to the present invention; and

FIG. 8 is a view illustrating a sectional structure of a light-emittingelement mounting a light-emitting element on the wiring board forlight-emitting element of FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

[Wiring Board for Light-Emitting Element]

In FIGS. 1(a) and 1(b) illustrating a representative structure of awiring board for light-emitting element of the present invention, thewiring board generally designated at 11 includes a ceramic insulatingsubstrate 1, conductor layers (connection terminals) 3 a and 3 b formedon one surface 1 a of the insulating board 1, conductor layers (externalelectrode terminals) 5 formed on the other surface 1 b of the insulatingsubstrate 1, and via-conductors 7 formed through the insulatingsubstrate 1 so as to electrically connect the conductor layers 3 a, 3 bto the conductor layers 5.

On the surface 1 a of the insulating substrate 1, there is formed amounting region 9 between one conductor layer 3 a and another conductorlayer 3 b for mounting a light-emitting element that will be describedlater. That is, the conductor layers 3 a and 3 b are electricallyconnected to the light-emitting element (not shown in FIGS. 1(a) and1(b)) mounted on the mounting region 9, and work as connectionterminals. Further, the conductor layers 5 are electrically connected toan external circuit board such as a printed board and work as externalelectrode terminals. As will be described later, therefore, thelight-emitting element mounted on the mounting region 9 is electricallyconnected to the external circuit board through the conductor layers 3a, 3 b, via-conductors 7 and conductor layers 5. In the followingdescription, therefore, the conductor layers 3 a, 3 b are calledconnection terminals, and the conductor layers 5 are called externalelectrode terminals.

The above connection terminals 3 a, 3 b and the external electrodeterminals 5 are formed by using various metals, and are, usually, formedby using at least-one of W, Mo, Cu or Ag as a chief component. When theconnection terminals 3 a, 3 b and the external electrode terminals 5 aare formed by using such a metal material, there is obtained anadvantage in that they can be formed by the co-firing with theinsulating substrate 1. Besides, the wiring board 11 for light-emittingelement can be produced inexpensively and quickly.

In the present invention as illustrated in FIG. 1(b), there can beprovided a frame 13 for containing the light-emitting element that ismounted so as to surround the mounting region 9 and the conductor layers3 a, 3 b. The frame 13 protects the light-emitting element mounted onthe mounting region 9, and makes it possible to easily arrange afluorescent body or the like surrounding the light-emitting element. Itis further allowed to reflect light of the light-emitting element by theframe 13 to guide it in a predetermined direction. It is desired thatthe total reflection factor of the inner wall of the frame 13 is notsmaller than 70%, particularly, not smaller than 80% and, mostpreferably, 85% from the standpoint of suppressing the transmission orabsorption of light from the light-emitting element and maintaining ahigh brightness.

[Heat-Conducting Pole-Like Conductor 10]

In the present invention, it is important that the mounting region 9 isprovided with the heat-conducting pole-like conductor 10 which isextending through the insulating substrate 1. The heat-conductingpole-like conductor 10 is formed by using a material having a thermalconductivity higher than that of the ceramics forming the insulatingsubstrate and, concretely, by using a metal such as W, Mo, Cu or Ag (oris often formed by using a composite material of a metal and ceramics aswill be described later). As shown in FIGS. 1(a) and 1(b), theheat-conducting pole-like conductor 10 extends from the mounting region9 up to the surface 1 b on the opposite side of the insulating substrate1 penetrating through the insulating substrate 1 in the direction ofthickness thereof. The heat-conducting pole-like conductor 10 is formedby the co-firing with the insulating substrate 1. Namely, in the wiringboard of the invention, the pole-like conductor 10 having a thermalconductivity higher than that of the insulating substrate 1 extends fromthe mounting region 9 through the insulating substrate 1 enabling theheat generated by the light-emitting element to be quickly radiatedthrough the pole-like conductor 10 which serves as a heat conductingpassage and, hence, preventing the light-emitting element from beingexcessively heated and preventing a decrease in the brightness of thelight-emitting element.

Referring to FIGS. 1(a) and 1(b), the pole-like conductor 10 is of theform of a block having a diameter of as large as 500 μm or greater, andis disposed in a number of one or in a plural number in a manner thatthe upper end surfaces thereof are positioned on the mounting region 9.The pole-like conductor 10 may have any one of a circular shape, anelliptic shape, a square shape or a polygonal shape in plane crosssection.

The pole-like conductor 10 of the shape of a block of a large diameteris formed by pushing and fitting a conductor sheet prepared by using aconducting slurry for forming the pole-like conductor 10 into theceramic green sheet in a manner to penetrate through the ceramic greensheet, and by co-firing the thus prepared composite molded article (thismethod will be described later in detail). The pole-like conductor 10 ofthe form of a block has a size like that of, for example, thelight-emitting element that is to be mounted, i.e., has a diameter inexcess of, for example, 1000 μm, and is capable of realizing a very highheat-radiating performance as compared to that of the one which is aso-called thermal via. To obtain a high heat-radiating performance, inparticular, it is desired that the plane sectional area of the pole-likeconductor 10 of the form of a block (area of the end surface on the sideof the mounting region 9) is greater than the mounting area (correspondsto the bottom surface area) for mounting the light-emitting element onthe wiring board 11 and is, for example, not smaller than 1.1 times asgreat and, most preferably, not smaller than 1.2 times as great as themounting area for mounting the light-emitting element. An increase inthe plane sectional area of the pole-like conductor of the form of ablock is accompanied by an increase in the heat-radiating portion makingit possible to more quickly radiate the heat generated by thelight-emitting element.

The pole-like conductor 10 illustrated in FIGS. 1(a) and 1(b) haselectric conductivity and can be incorporated in a portion of theelectric circuit. For example, upon directly and electrically connectingthe light-emitting element that is mounted and the pole-like conductor10 together, the connection terminals 3 a, 3 b and the via-conductors 7are no longer required, and the wiring board 11 for light-emittingelement can be realized in a small size. When the heat-conductingpole-like conductor 10 is provided independently of the electriccircuit, there is no electric connection between the heat-conductingpole-like conductor 10 and the external circuit board such as a printedboard, and the mounting reliability is improved.

In the present invention, the above-mentioned pole-like conductor 10 hasa thermal conductivity higher than that of the insulating substrate and,particularly, has a thermal conductivity of not smaller than 80 W/m·K,preferably, not smaller than 100 W/m·K, more preferably, not smallerthan 120 W/m·K, and, most preferably, not smaller than 160 W/m·K. Uponproviding the pole-like conductor 10 having a high thermal conductivity,the heat generated by the light-emitting element can be directly andquickly radiated making it possible to maintain stable the emission oflight from the light-emitting element, which is desirable from thestandpoint of preventing a decrease in the brightness of thelight-emitting element. The thermal conductivity of the pole-likeconductor 10 can be controlled by suitably selecting the kind of themetal forming it. For example, the pole-like conductor 10 formed byusing Cu or Cu—W having a high thermal conductivity exhibits anincreased thermal conductivity. To maintain a high thermal conductivityas described above, it is desired that the Cu content in the pole-likeconductor 10 is not smaller than 40% by volume, particularly, notsmaller than 50% by volume and, more particularly, not smaller than 60%by volume.

In order to improve the reliability of adhesion between the pole-likeconductor 10 and the insulating substrate 1, further, it is desired thatthe coefficient of thermal expansion (40 to 400° C.) of the pole-likeconductor 10 is brought close to the coefficient of thermal expansion ofthe insulating substrate 1. For example, it is desired that thecoefficient of thermal expansion of the pole-like conductor 10 is soadjusted that a difference Δα in the coefficient of thermal expansionbetween the insulating substrate 1 and the pole-like conductor 10 is notlarger than 4.0×10⁻⁶/° C., preferably, not larger than 2.0×10⁻⁶/° C.and, most preferably, not larger than 1.0×10⁻⁶/° C. This is because thehighly reliable wiring board 11 for light-emitting element is obtainedupon preventing the mismatching in the thermal expansion between thetwo. The coefficient of thermal expansion of the pole-like conductor 10can be adjusted by suitably selecting the kind of the metal. By using,for example, Cu having a high thermal conductivity in combination with Whaving a coefficient of thermal expansion which is relatively smallamong the metals, it is allowed to form the pole-like conductor 10having a high thermal conductivity and exhibiting a suppresseddifference in the coefficient of thermal expansion from the insulatingsubstrate 1. Upon forming the pole-like conductor 10 by using acomposite material of a metal and ceramics, further, it is allowed toadjust the difference in the coefficient of thermal expansion betweenthe two.

In the present invention, the above-mentioned pole-like conductor 10 isprepared by mixing a predetermined metal powder (e.g., powder of Cu or Wdescribed above) with suitable amounts of an organic binder and anorganic solvent. So far as a thermal conductivity higher than that ofthe insulating substrate 1 is maintained, a mixed powder of the metalpowder and the ceramics powder (e.g., powder of ceramics for forming theinsulating substrate 1) may be prepared being mixed with the organicbinder and the organic solvent. So far as the conducting paste of theabove mixed powder is used, a high adhering strength is accomplishedbetween the pole-like conductor 10 and the insulating substrate 1through the co-firing with the ceramic green sheet, and thepole-like-conductor 10 is formed by using a composite material of ametal and ceramics. In this case, further, the mixing ratio of the metalpowder and the ceramic powder is adjusted to easily control thecoefficient of thermal expansion, to bring the coefficient of thermalexpansion of the pole-like conductor 10 close to that of the insulatingsubstrate 1, and to suppress the occurrence of cracks caused bymismatching in the thermal expansion between the insulating substrateand the pole-like conductor 10. It is desired that the content of theceramic powder (corresponds to the content of ceramics in the pole-likeconductor 10) in the mixed powder is, usually, not larger than 5% byvolume, particularly, not larger than 4% by volume and, most desirably,not larger than 3% by volume to maintain a high thermal conductivity. Asthe ceramics, it is most desired to use those ceramics that have beenused for forming the insulating substrate 1, as a matter of course.

In the present invention as shown in FIG. 2, further, there can beprovided boundary protection layers 15 covering the boundary portionsbetween the end surfaces of the pole-like conductor 10 and theinsulating substrate 1 and covering the vicinities thereof. The boundaryprotection layers 15 are formed by using at least one of a metal,ceramics or a resin. Provision of the boundary protection layers 15relaxes the difference in the thermal expansion between the pole-likeconductor 10 and the insulating substrate 1, and suppresses theoccurrence of cracks in the boundary.

The composition of the boundary protection layer 15 can be suitablyvaried by taking into consideration the difference in the thermalexpansion between the pole-like conductor 10 and the insulatingsubstrate 1. By using, for example, a metal or ceramics, the boundaryprotection layer 15 can be formed by the co-firing with the insulatingsubstrate 1 and the pole-like conductor 10. It is desired that the aboveceramics have the same composition as the ceramics used for forming theinsulating substrate 1 from the standpoint of sintering property andadhesiveness between the boundary protection layer 15 and the insulatingsubstrate 1. As the above metal, there can be used the one having thesame composition as that of the pole-like conductor 10 to improve thesintering property and the adhesiveness between the boundary protectionlayer 15 and the pole-like conductor 10. From the standpoint ofmaintaining reliability to the heat cycle, however, it is desired to usea metal having a coefficient of thermal expansion smaller than that ofthe insulating substrate 1. When, for example, the insulating substrate1 is made of alumina, it is desired that Cu—W or Mo is used as a metalfor forming the boundary protection layer. When the boundary protectionlayer 15 is used by using a metal, further, it is desired to useceramics in combination since it makes it possible to control thebehavior of sintering and the coefficient of thermal expansion.

When a resin is used as the boundary protection layer 15, the insulatingsubstrate 1 and the pole-like conductor 10 are formed by co-firing.Thereafter, the resin is so printed as to cover the boundary between theinsulating substrate 1 and the pole-like conductor 10 followed by curingthereby to form the boundary protection layer 15. When the resin is usedas the boundary protection layer 15, there is contained a ceramic powderin an amount of 10 to 50% by volume in addition to the resin componentto improve the water resistance and heat-radiating performance of theboundary protection layer 15.

The above boundary protection layer 15 may have a laminated-layerstructure consisting of layers of different materials. When, forexample, the surface protection layer 15 is formed by laminating a resinlayer on the layer formed by using metals and ceramics, cracks thathappen to occur on the boundary between the insulating substrate 1 andthe pole-like conductor 10 are prevented from developing into thesurface layer. Therefore, the above laminated-layer structure is mostdesired.

Reference should further be made to FIGS. 3(a) and 3(b) illustratinganother embodiment of the wiring board 11 for light-emitting element ofthe present invention.

In the present invention as shown in, for example, FIG. 3(a), a coveringlayer 16 a can be so formed as to completely cover the end surface ofthe heat-conducting pole-like conductor 10 of the side on where themounting region 9 is formed and the peripheral edge thereof. Thecovering layer 16 a formed on the end surface of the upper side containsa resin or a metal. Formation of the covering layer 16 a, too, relaxesthe difference in the thermal expansion between the pole-like conductor10 and the insulating substrate 1, and suppresses the occurrence ofcracks on the boundary between the end surface of the pole-likeconductor 10 and the insulating substrate 1. The covering layer 16 a,further, works to fix a light-emitting element 21 that will be describedlater to the mounting region 9. In particular, the covering layer 16 aformed by using a metal exhibits a thermal conductivity higher than thatof the insulating substrate 1 making it possible to quickly release theheat from the light-emitting element 21. Further, the covering layer 16a formed by using a resin prevents the short-circuit between thepole-like conductor 10 and the connection terminals 3 a, 3 b.

Referring to FIG. 3(b), further, it is also allowable to so form acovering layer 16 b as to completely cover the end surface of thepole-like conductor 10 positioned on the side opposite to the side onwhere the mounting region 9 is formed and the peripheral edge thereof.The covering layer 16 b formed on the end surface of the lower sidecontains at least one of those selected from the group consisting of ametal, ceramics and a resin. Formation of the covering layer 16 b, too,relaxes the difference in the thermal expansion between the pole-likeconductor 10 and the insulating substrate 1, and suppresses theoccurrence of cracks on the boundary between the end surface of thepole-like conductor 10 and the insulating substrate 1. Further, thecovering layer 16 b formed by using a metal exhibits a thermalconductivity higher than that of the insulating substrate 1 making itpossible to quickly release the heat from the light-emitting element 21.Further, the covering layer 16 b formed by using ceramics or a resinprevents the short-circuit between the pole-like conductor 10 and theexternal terminals 5. When ceramics is used, in particular, the coveringlayer 16 b can be formed by co-firing with the insulating substrate 1and the pole-like conductor 10, offering advantage from the standpointof production.

When the covering layer 16 a is formed on the upper surface by using aresin and a covering layer 16 b is formed on the lower surface by usingceramics or a resin as will be understood from the above description, awiring can be arranged just under the pole-like conductor 10 at the timeof mounting, on a printed board, a light-emitting device which isobtained by mounting the light-emitting element 21 on the wiring board11, offering an advantage from the standpoint of realizing the devicesin small sizes.

The above covering layers 16 a and 16 b can be easily formed by applyinga paste containing a metal or ceramics or by applying a coating solutioncontaining a resin onto the above portions followed by firing or baking.Further, the metal or ceramics used for forming the covering layers 16a, 16 b may be the same as the one used for forming the boundaryprotection layer 15 described above.

In the present invention, further, the side surface of the pole-likeconductor 10 may be tilted or a step may be formed in the side surfaceto enhance the junction strength between the pole-like conductor 10 (inparticular, the pole-like conductor 10 of the form of a block having alarge diameter) and the insulating substrate 1, so that the pole-likeconductor 10 is firmly incorporated in the insulating substrate 1.

As shown in FIG. 4(a), for example, the side surface of the pole-likeconductor 10 is tilted as designated at 10 a to increase the contactarea between the pole-like conductor 10 and the insulating substrate 1thereby to increase the junction strength between the insulatingsubstrate 1 and the pole-like conductor 10. In the illustratedembodiment, the end surface of the pole-like conductor 10 on the side ofthe mounting region 9 is smaller than the other end surface, so that theside surface thereof is tilted. It is, however, also allowable to formthe end surface of the pole-like conductor 10 on the side of themounting region 9 to be greater than the other end surface, thereby toform a tilted surface. From the standpoint of heat-radiatingperformance, however, it is desired that the end surface of thepole-like conductor 10 on the side of the mounting region 9 is smallerthan the other end surface.

Referring to FIG. 4(b), further, a step 10 b may be formed on the sidesurface of the pole-like conductor 10. In this case, too, the contactarea between the pole-like conductor 10 and the insulating substrate 1is greater than that of when the side surface of the pole-like conductor10 is straight, enhancing the junction strength between the insulatingsubstrate 1 and the pole-like conductor 10. In this illustratedembodiment, further, the end surface of the pole-like conductor 10 onthe side of the mounting region 9 is smaller than the other end surface.Contrary to this, however, a step may be so formed that the end surfaceof the pole-like conductor 10 on the side of the mounting region 9 isgreater than the other end surface. From the standpoint ofheat-radiating performance, however, it is desired that the step 10 b isso formed that the end surface of the pole-like conductor 10 on the sideof the mounting region 9 is smaller than the other end surface.

In the embodiment of FIG. 4(b), further, only one step 10 b is formed onthe side surface of the pole-like conductor 10. As shown in FIG. 4(c),however, a plurality of steps 10 b may be formed. That is, in FIG. 4(c),there are formed two steps 10 b on the side surface of the pole-likeconductor 10 so that a protruded portion 10 c is formed on the sidesurface of the pole-like conductor 10. This increases the contact areabetween the pole-like conductor 10 and the insulating substrate 1 and,further, enables the side surface of the pole-like conductor 10 tobecome in firm mesh with the insulating substrate 1. In this embodiment,the junction strength between the two is very enhanced. In FIG. 4(c),the protruded portion 10 c is formed on the side surface due to twosteps 10 b. It is also possible to form a recessed portion in the sidesurface relying on the two steps 10 b, as a matter of course. From thestandpoint of heat-radiating performance, however, it is desired to formthe protruded portion 10 c.

In the above embodiment of FIGS. 4(b) and 4(c), it is desired that thelength L of the step 10 b is, usually, not smaller than 100 μm and,particularly, not smaller than 200 μm from the standpoint of increasingthe junction strength.

In the embodiment of FIGS. 4(a) to 4(c), it is desired that the planesectional area of the pole-like conductor 10 (particularly, the area ofthe end surface on the side of the mounting region 9) is greater thanthe mounting area for mounting the light-emitting element from thestandpoint of maintaining a high heat-radiating performance.

When the steps 10 b are formed on the side surface of the pole-likeconductor 10 as shown in FIGS. 4(b) and 4(c), further, it is desiredthat a conducting layer 14 is drawn from the step 10 b as shown in FIG.5. That is, when the step 10 b is formed on the side surface of thepole-like conductor 10, stress generates due to a difference in thethermal expansion between the insulating substrate 1 and the pole-likeconductor 10 and concentrates near the step 10 b. As a result, when thelight-emitting element that is mounted is repetitively operated, crackstend to occur near the step 10 b due to heat generated by the emissionof light. On the other hand, formation of the conductor layer 14 relaxesthe concentration of stress in the portion near the step 10 b andeffectively suppresses the generation of cracks. The conducting layer 14is formed by using the same material as the one used for the pole-likeconductor 10 offering an advantage of further improving theheat-radiating performance.

In FIG. 5, further, when the step 10 b is formed on the side surface ofthe pole-like conductor 10, the insulating substrate 1, usually, assumesa laminated-layer structure of a lamination of a plurality of insulatinglayers (a two-layer structure of insulating layers 1 a and 1 b in FIG.5), and the step 10 b is formed on the interface of lamination of theinsulating layers 1 a and 1 b (refer to a method of forming thepole-like conductor 10 described later). Therefore, the conducting layer14 extends from the edge of the step 10 b along the interface oflamination of the insulating layers 1 a, 1 b. In this case, it isdesired that the length w of protrusion from the edge of the step 10 bof the conducting layer 14 is not smaller than 50 μm, particularly, isnot smaller than 200 μm and, most desirably, is not smaller than 400 μmto relax the stress to a sufficient degree. Further, the conductinglayer 14 is formed by using the same material as the one forming thepole-like conductor 10 and may, hence, extend through the pole-likeconductor 10 traversing the pole-like conductor 10 as shown in FIG. 5.

FIG. 5 illustrates the formation of the conducting layer 14 in a casewhere a single step 10 b is formed on the side surface of the pole-likeconductor 10. When a plurality of steps 10 b are formed as shown in FIG.4(c), however, it is desired that the conducting layers 14 are drawnfrom the plurality of steps 10 b.

[Production of the Insulating Substrate 1 and the Wiring Substrate 11]

It was described already that the insulating substrate 1 in the presentinvention is made of ceramics. It is desired that the insulatingsubstrate 1 has a thermal conductivity of not smaller than 30 W/m·K,preferably, not smaller than 35 W/m·K, more preferably, not smaller than40 W/m·K and, most preferably, not smaller than 45 W/m·K. The higher thethermal conductivity, the larger the heat-radiating performance from theinsulating substrate 1 and the greater the effect for suppressing adecrease of brightness of the light-emitting element. By using, forexample, highly pure alumina having a purity of not smaller than 99% asceramic materials, there can be produced an insulating substrate havinga thermal conductivity of not smaller than 30 W/m·K. By using MgO,further, there can be produced an insulating substrate 1 having athermal conductivity of not smaller than 40 W/m·K.

It is further desired that the coefficient of thermal expansion of theinsulating substrate 1 (room temperature to 400° C.) is not smaller than8.5×10⁻⁶/° C., preferably, not smaller than 9.0×10⁻⁶/° C. and, mostpreferably, not smaller than 10.0×10⁻⁶/° C. That is, the difference inthe thermal expansion between the insulating substrate 1 and thepole-like conductor 10 or the printed board mounted on the outer sidecan be decreased by increasing the coefficient of thermal expansion ofthe ceramic insulating substrate 1, and the connection reliability canbe strikingly improved between the insulating substrate 1 and thepole-like conductor 10 or the printed board. It is further allowed toimprove the reliability of connection to the resin used for sealing thelight-emitting element that is mounted. For example, use of forsteriteas the ceramic material makes it possible to produce the insulatingsubstrate 1 having a coefficient of thermal expansion of not smallerthan 8.5×10⁻⁶/° C. and use of MgO makes it possible to obtain theinsulating substrate 1 having a coefficient of thermal expansion of notsmaller than 10.0×10⁻⁶/° C.

It is further desired that the insulating substrate 1 has a totalreflection factor of not smaller than 70%, preferably, not smaller than72%, more preferably, not smaller than 80% and, most preferably, notsmaller than 83%. Upon increasing the reflection factor, it is madepossible to suppress the light of the light-emitting element fromtransmitting through the insulating substrate 1, to suppress the emittedlight from being absorbed by the insulating substrate 1, and to obtainthe wiring board 11 for light-emitting element that has a goodlight-emitting efficiency. The insulating substrate 1 having areflection factor of as high as 83% or more can be produced by, forexample, using highly pure alumina having a purity of not lower than 99%or by using MgO to which Y₂O₃ is added as a sintering assistant.

It is further desired that the three-point flexural strength of theinsulating substrate 1 is not smaller than 350 MPa, particularly, notsmaller than 400 MPa and, most preferably, not smaller than 450 MPa. Thehighly strong insulating substrate 1 is not cracked by stress at thetime when the light-emitting device mounting the light-emitting elementon the wiring board 11, is mounted on an external circuit board such asa printed board. The highly strong insulating substrate 1 can beobtained by using alumina or MgO as the ceramic material.

As will be understood from the foregoing description, the ceramicmaterial used for forming the insulating substrate 1 should be selectedby taking into consideration the above-mentioned thermal conductivity,coefficient of thermal expansion and other properties. For example, theinsulating substrate 1 made of MgO comprises a sintered body of MgOcontaining MgO as a main crystal phase, exhibiting a coefficient ofthermal expansion (room temperature to 400° C.) of as high as about10×10⁻⁶/° C., enhancing reliability of mounting on a general-purposeprinted board (coefficient of thermal expansion of not smaller than10×10⁻⁶/° C. or higher), exhibiting a thermal conductivity of notsmaller than 30 W/m·K as well as further increased total reflectionfactor and three-point flexural strength.

The sintered body of MgO containing MgO as a main crystal phase is theone of which the peak of MgO can be detected as a main peak by, forexample, the X-ray diffraction and which contains MgO crystals in anamount of not less than 50% by volume as a volume ratio.

The insulating substrate 1 of MgO is obtained by firing a ceramic greensheet at a temperature of higher than 1050° C. and, particularly, in atemperature range of 1300 to 1700° C., the ceramic green sheet beingobtained by molding a mixed-powder of an MgO powder having an averageparticle size in a range of 0.1 to 8 μm and a purity of not lower than99% and at least one kind of a sintering assistant selected from thegroup consisting of a rare earth oxide (e.g., Y₂O₃, Yb₂O₃, etc.), Al₂O₃,SiO₂, CaO, SrO, BaO, B₂O₃ and ZrO₂ or a filler powder (average particlesize of 0.1 to 8 μm). To the above mixed powder, further, there may beadded MgAl₂O₄ containing MgO or an MgO.SiO₂ composite oxide.

It is desired that the additive such as the sintering assistant is usedfor obtaining a densely sintered body with MgO as a main crystal, beingadded in an amount of not less than 3% by mass and, particularly, notless than 5% by mass to lower the firing temperature. From thestandpoint of precipitating the MgO crystals in large amounts, further,it is desired that the above additive is added to the mixed powder in anamount of not lager than 30% by mass and, particularly, not larger than20% by mass. When the amount of the additive is not larger than 10% bymass, in particular, most of the insulating substrate 1 that is obtainedis formed by the MgO crystals.

The insulating substrate 1 made of the alumina comprises a sintered bodyof Al₂O₃ containing Al₂O₃ as a main crystal phase, exhibits propertiesas described above and further offers an advantage of low cost. Thesintered body of Al₂O₃ containing Al₂O₃ as a main crystal phase is theone of which the peak of Al₂O₃ can be detected as a main peak by, forexample, the X-ray diffraction and which contains Al₂O₃ crystals in anamount of not less than 50% by volume.

The insulating substrate 1 of alumina is obtained by firing a ceramicgreen sheet at a temperature of not higher than 1050° C. and,particularly, in a temperature range of 1300 to 1500° C., the ceramicgreen sheet being obtained by molding a mixed powder of an Al₂O₃ powderhaving an average particle size of 1.0 to 2.0 μm and a purity of notlower than 99% to which is added at least one kind of a sinteringassistant powder (average particle size of 1.0 to 2.0 μm) selected fromthe group consisting of Mn₂O₃, SiO₂, MgO, SrO, and CaO.

In producing the alumina insulating substrate 1 in a manner as describedabove, it is desired that the additive such as the sintering assistantis added to the mixed powder for molding the ceramic green sheet in anamount of not less than 5% by mass and, particularly, not less than 7%by mass to lower the firing temperature. From the standpoint ofobtaining a densely sintered body containing Al₂O₃ as main crystals,further, it is desired that the above additive is added to the mixedpowder in an amount of not lager than 15% by mass and, particularly, notlarger than 10% by mass. In this case, most of the insulating substrate1 that is obtained comprises Al₂O₃ crystals.

In the foregoing were described the MgO insulating substrate 1 and thealumina insulating substrate 1. However, the insulating substrate usedin the present invention is not limited thereto only but may further usesintered bodies containing mullite, spinel or forsterite as maincrystals of ceramics. It is further allowable to form the insulatingsubstrate 1 using the so-called glass ceramics. The glass ceramicinsulating board 1 can be produced by firing at a low temperature of nothigher than 1050° C. featuring a dense and smooth surface, particularly,in forming the connection terminals 3 a, 3 b, external electrodeterminals 5 and via-conductors 7 by using a conductor of a lowresistance, such as Ag or Cu relying on the co-firing.

The glass ceramic insulating substrate 1 can be produced by firing aceramic green sheet at a temperature of not higher than 1050° C. and,particularly at a temperature of 850° C. to 1050° C., the ceramic greensheet being obtained by molding, in the same manner as described above,a mixed powder of, for example, a glass powder and a filler powder suchas SiO₂ powder. To produce the highly strong insulating substrate 1, itis desired that the content of the filler in the mixed powder is,usually, in a range of 30 to 60% by mass and, particularly, 35 to 55% bymass though it may vary depending upon the composition of the glass.

In producing the above various insulating substrates 1, the ceramicgreen sheet can be molded by a known method by using a mixed powder ofstarting materials. For instance, a slurry for molding is prepared byadding a binder and a solvent to a mixed powder containing a ceramicmaterial such as MgO or Al₂O₃. The slurry is, then, molded into aceramic green sheet by such means as a doctor blade method.

To produce the wiring board 11, therefore, through holes correspondingto the via-conductors 7 are perforated in the ceramic green sheet atpredetermined positions by the laser working, etc., and a conductingpaste obtained by dispersing a metal powder in a suitable binder or asolvent is filled in the through holes. Further, the conducting paste isprinted on the surface of the ceramic green sheet at predeterminedpositions in a pattern that corresponds to the connection terminals 3 a,3 b and the external electrodes 5. Moreover, the conducting patterncorresponding to the pole-like conductor 10 is incorporated in theceramic green sheet and is fired (co-fired) in this state, whereby theconnection terminals 3 a, 3 b and the via-conductors 7 are formed on thesurface and inside the insulating substrate 1, and there is obtained thewiring board 11 provided with the pole-like conductor 10. The connectionterminals 3 a, 3 b and the external electrode terminals 5 can bedirectly formed on the surface of the insulating substrate bytransferring a metal foil onto the ceramic green sheet or by a thinfilm-forming method such as vaporization.

The conducting pattern corresponding to the pole-like conductor 10 isincorporated into the ceramic green sheet in a manner as describedbelow.

When the pole-like conductor 10 has a shape as shown in FIG. 1(a),through holes are perforated in the green sheet at predeterminedposition like the case of forming the via-conductors 7, and theconducting paste containing a metal powder for forming the pole-likeconductor 10 is filled in the through holes, so that the conductingpattern for forming the pole-like conductor 10 is incorporated in theceramic green sheet.

When the pole-like conductor 10 has the shape of a block as shown inFIG. 1(b), the conducting sheet corresponding to the pole-like conductor10 is pushed into the ceramic green sheet at predetermined positions soas to incorporate the conducting pattern for forming the pole-likeconductor 10. FIG. 6 illustrates a method of forming the conductingpattern.

That is, referring to FIG. 6(a), a ceramic green sheet 40 is arranged onthe upper surface of a metal mold 39 having a punched hole 37.

Referring, next, to FIG. 6(b), a conducting sheet 43 for forming thepole-like conductor 10 is overlapped on the ceramic green sheet 40.Desirably, the conducting sheet 43 has a thickness nearly comparable tothat of the ceramic green sheet 40. The conducting sheet 43 is preparedby molding, relying on a sheet-molding method such as the doctor blademethod, a conducting slurry obtained by mixing a metal powder forforming the above pole-like conductor 10 (a mixed powder of the metalpowder and the ceramic powder) into an organic binder and a solvent.

Referring, next, to FIG. 6(c), the conducting sheet 43 is pushed intothe ceramic green sheet 40 by using a pushing metal mold 35. There is,thus, established a state where part of the conducting sheet 43 isinserted in the ceramic green sheet 43.

Further, the conducting sheet 43 of a portion that has not been pushedinto the ceramic green sheet 43 is removed and, at the same time, aportion of the ceramic green sheet 40 pushed by the conducting sheet 43is removed to form a composite sheet 50 in which part of the conductingsheet 43 is incorporated so as to penetrate through part of the ceramicgreen sheet 40 as shown in FIG. 6(d). Namely, the conducting sheet 43fitted into the composite sheet 50 as shown in FIG. 6(d) becomes aconducting pattern corresponding to the pole-like conductor 10. On theabove composite sheet (green sheet) 50, there are formed conductingpatterns corresponding to the connection terminals 3 a, 3 b, externalterminals 5 and via-conductors 7 as described above. The composite sheet(green sheet) 50 in this state is subjected to the firing.

In order to form the pole-like conductor 10 having a tilted side surface10 a as shown in FIG. 4(a), the punched hole 37 may have a taperedsurface that corresponds to the tilted surface 10 a.

To form the pole-like conductor 10 having a step 10 b formed on the sidesurface as shown in FIG. 4(b), there are prepared, as shown in FIG. 7, acomposite sheet 50 a incorporating therein a conducting sheet 43 of asmall diameter and a composite sheet 50 b incorporating therein aconducting sheet 43 of a large diameter according to the above-mentionedmethod, and the composite sheets 50 a and 50 b are press-adhered toprepare a laminate thereof. In this laminate, a step 10 b is formed atan end of a portion where the conducting sheet 43 of the small diameterformed in the composite sheet 50 a faces the conducting sheet 43 of thelarge diameter formed in the composite sheet 50 b. Upon firing thelaminate, therefore, the pole-like conductor 10 of the shape shown inFIG. 4(b) is formed in the insulating substrate 1. In this case, theinsulating substrate 1 has a laminated-layer structure of two insulatinglayers laminated one upon the other. To form the conducting layer 14shown in FIG. 5, further, the conducting paste may be applied, byscreen-printing or the like, onto the interface of lamination of eitherthe composite sheet 50 a or the composite sheet 50 b as shown in FIG. 5so as to be corresponded to the conducting layer 14.

To form the pole-like conductor 10 having a plurality of steps 10 bformed on the side surface as shown in FIG. 4(c), further, the compositesheet 50 b incorporating therein the conducting sheet 43 of a largediameter is held by two pieces of composite sheets 50 a (incorporatingtherein the conducting sheet 43 of a small diameter) to prepare alaminate of a three-layer constitution, which is, then, fired. In thiscase, the insulating substrate 1 that is fabricated has alaminated-layer structure of three insulating layers.

In the embodiment of FIG. 7, the laminate of the composite sheet 50 isprepared for forming the pole-like conductor 10 having the step 10 b. Toform the pole-like conductor 10 without step 10 b, on the other hand,there is prepared a laminate of the composite sheets 50 which is, then,fired. In this case, the compositions of the conducting sheets 43 in thecomposite sheets 50 are differed to form the pole-like conductor 10comprising a plurality of layers having coefficients of thermalexpansion and thermal conductivities which are different from eachother. For example, there is formed the pole-like conductor 10 bylaminating a first composite sheet burying therein a conducting sheetcomprising 40% by volume of Cu and 60% by volume of W, and a secondcomposite sheet burying therein a conducting sheet comprising 50% byvolume of Cu and 50% by volume of W in a manner that the first compositesheet is on the side of the mounting region 9 and the second compositesheet is on the side opposite to the mounting region 9. In this case,the layer (side of the mounting region 9) stemming from the conductingsheet of the first composite sheet has a low coefficient of thermalexpansion creating a small difference in the coefficient of thermalexpansion from the light-emitting element that is mounted while thelayer (side opposite to the mounting region 9) stemming from theconducting sheet in the second composite sheet has a large thermalconductivity yet exhibiting a large coefficient of thermal expansion. Aswill be understood from the above, if the layer on the side of themounting region 9 is the one exhibiting a small coefficient of thermalexpansion, the layer having a high thermal conductivity can be arrangedon the side opposite to the mounting region 9 without the need of givingattention to the coefficient of thermal expansion making it possible tomarkedly enhance the heat-radiating performance yet maintainingreliability of connection to the light-emitting element.

Further, a paste containing a metal, such as an Mo paste, is applied byscreen-printing or the like method onto the exposed surface of theconducting sheet 43 (corresponds to the end surface of the pole-likeconductor 10) in the composite sheet 50 (or in the laminate of thecomposite sheets 50) and onto the interface between the green sheet 40and the conducting sheet 43, and is fired to form the covering layers 16a, 16 b shown in FIGS. 3(a) and 3(b) on the end surface of the upperside of the pole-like conductor 10 or on the end surface of the lowerside thereof. The boundary protection layers 15 shown in FIG. 2 areformed by applying a paste having the same composition as that of theabove green sheet 40 onto the predetermined positions of the compositesheet like the above-mentioned metal paste, followed by firing. Further,to form the covering layers 16 a, 16 b and the boundary protectionlayers 15 by using a resin, a coating solution containing the resin isapplied onto the predetermined positions after the composite sheet hasbeen sintered, and is dried and cured.

The above ceramic green sheet (or composite sheet) is fired by beingheated at a predetermined firing temperature in an oxidizing atmosphere,a reducing atmosphere or an inert atmosphere after the binder has beenremoved. In particular, when a material subject to be oxidized like Cuis used as the metal powder, the firing is conducted in the reducingatmosphere or in the inert atmosphere.

After the firing, as required, Al or Ag is plated on the surfaces of theconnection terminals 3 a, 3 b, external electrode terminals 5 andpole-like conductor 10 to increase the reflection factor of thesemembers and the brightness. In particular, nickel, gold and silver areplated in this order on the metallized surfaces of the connectionterminals 3 a, 3 b, pole-like conductor 10 and inner wall surfaces 13 aof the frame to firmly fix a silver-plated layer having a highreflection factor. Upon forming the silver-plated layer on the innersurfaces 13 a of the frame, in particular, light from the light-emittingelement is reflected by the inner wall surfaces 13 a and is emitted tothe outer side.

In forming the nickel-plated layer as the underlying layer, if silver isdirectly plated on the nickel-plated layer, nickel in the nickel-platedlayer elutes into the silver-plating bath and deposits on the wiringconductor and on the silver-plated layer on the mounting region, ornickel particles are peeled off the nickel-plated layer due to vibrationduring the conveyance before the silver-plating treatment and depositson the wiring conductor and on the mounting region. In this embodiment,however, gold is plated on the nickel-plated layer without arousing theabove inconvenience, without hindering the mounting of thelight-emitting element and suppressing a drop in the bonding property.

In the present invention, the frame 13 for protecting the light-emittingelement that is mounted is formed by using a ceramic material or a metalmaterial. When the ceramics is used, the frame 13 can be formed at onetime by co-firing together with the above insulating substrate 1,connection terminals 3 a, 3 b, external terminals 5 and pole-likeconductor 10 offering advantage from the standpoint of productivity.Besides, the ceramic frame 13 features excellent heat resistance andmoisture resistance, exhibiting excellent durability even when used forextended periods of time or under severe conditions. On the inner wallsurface 13 a of the ceramic frame 13, further, a metallized layer (notshown) is formed by co-firing by using the conducting paste used forforming the above connection terminals 3 a, 3 b to further improve thedurability and to increase the reflection factor. On the metallizedlayer, there can be further formed a plated layer (not shown) of Ni, Auor Ag. Upon forming the metallized layer and the plated layer, the totalreflection factor of the inner wall surface 13 a of the frame 13 can beadjusted to be not lower than 70%, particularly, not lower than 80% and,most desirably, not lower than 85% to suppress the transmission orabsorption of light from the light-emitting element and, hence, torealize a high degree of brightness. Particularly, the inner wallsurface 13 a of the frame 13 having a reflection factor of as high as85% or more can be realized by forming a lustrous Ag plating.

Further, the metallic frame 13 has an advantage of having a highreflection factor by itself. As such a metal, it is desired to use Al oran Fe—Ni—Co alloy since it is inexpensive and can be excellently worked.On the inner wall surface 13 a of the metallic frame 13, too, there canbe plated a layer (not shown) of Ni, Au or Ag like the one describedabove. The above plated layer further increases the total reflectionfactor of the inner wall surface 13 a; i.e., helps realize the totalreflection factor of not lower than, for example, 85%. The metallicframe 13 is formed, for example, by forming a conducting layer 17 inadvance on the surface 1 a of the insulating substrate 1 obtainedthrough the firing, and brazing the conducting layer 17 and the frame 13together via a brazing material such as a eutectic Ag—Cu brazingmaterial.

In the embodiment of FIG. 1(b), the inner wall surface 13 a of the frame13 is represented by an erected surface. Not being limited to theerected surface, however, the inner wall surface 13 a may be in theshape of a flaring curved surface or a tilted surface of which thediameter is increasing toward the upper side. Forming the inner wallsurface 13 in a curved surface or a tilted surface is desirable from thestandpoint of guiding light of the light-emitting element toward theouter side.

[Light-Emitting Device]

The wiring board 11 for light-emitting element of the invention producedas described above can be used as a light-emitting device by mountingthe light-emitting element on the mounting region 9 thereof.

In FIGS. 8(a) and 8(b) which are sectional views illustrating thestructure, the light-emitting device generally designated at 25 has astructure in which a light-emitting element 21 such as an LED chip ismounted on the mounting region 9 of the wiring board 11. Thelight-emitting device 25 illustrated in FIG. 8(a) is the one mountingthe light-emitting element 21 on the wiring board 11 of FIG. 1(a), andthe light-emitting device 25 illustrated in FIG. 8(b) is the onemounting the light-emitting element 21 on the wiring board 11 of FIG.1(b).

In this light-emitting device 25, the light-emitting element 21 isadhered and fixed to the mounting region 9 of the wiring board 11 with asuitable adhesive 29 and is connected to the connection terminals 3 a, 3b of the wiring board 11 through the bonding wires 23. Namely, thelight-emitting element 21 works upon feeding electricity to thelight-emitting element 21 from the connection terminals 3 a, 3 b throughthe bonding wires 23. Further, the light-emitting element 21 can beconnected and fixed to the mounting region 9 by a so-called flip-chipconnection without using the adhesive 29. In this case, the electricitycan be directly fed to the light-emitting element 21 from, for example,the connection terminals 3 a, 3 b without using the bonding wires 23.

Further, the light-emitting element 21 mounted on the wiring board 11 issealed with a molding material 31 of a transparent resin material or thelike. However, the light-emitting element 21 can be sealed even by usinga closure member of a transparent material (e.g., glass) instead ofusing the above molding material. A fluorescent material may be added tothe molding material 31 to change the wavelength of light emitted by thelight-emitting element 21.

In the above light-emitting device 25, heat generated by thelight-emitting element 21 is quickly radiated from the heat-conductingpole-like conductor 10 making it possible to effectively avoid adecrease in the brightness caused by heat and, hence, to emit lightmaintaining stability and high brightness for extended periods of time.Besides, no heat-radiating member such as heat sink is necessary, whichis very effective in decreasing the size of the electric device on whichthe light-emitting device 25 is mounted.

Besides, light emitted from the light-emitting device 25 is reflected bythe surface of the insulating substrate 1 and by the inner surfaces ofthe frame 13 so as to be guided to a predetermined direction, enhancingthe light-emitting efficiency.

Moreover, the light-emitting device 25 is, usually, mounted on anexternal circuit board (not shown) such as a printed board through theexternal connection terminals 5. Here, the coefficient of thermalexpansion of the insulating substrate 1 is brought close to that of theprinted board to suppress mismatching in the coefficient of thermalexpansion relative to the printed board or the molding material 31,realizing the light-emitting device 25 featuring a highly reliablejunction.

1. A wiring board for light-emitting element, comprising a ceramicinsulating substrate, and a conductor layer formed on the surface or inthe inside of said insulating substrate, and having a mounting regionmounting a light-emitting element on one surface of said insulatingsubstrate; wherein said insulating substrate is provided with aheat-conducting pole-like conductor having a thermal conductivity higherthan that of said insulating substrate; and said heat-conductingpole-like conductor is extending through said insulating substrate inthe direction of thickness thereof from the light-emitting elementmounting region of said insulating substrate, and is formed by co-firingwith said insulating substrate.
 2. A wiring board for light-emittingelement according to claim 1, wherein said insulating substrate isformed by firing at a temperature of higher than 1050° C.
 3. A wiringboard for light-emitting element according to claim 1, wherein saidinsulating substrate is formed by firing at a temperature of not higherthan 1050° C.
 4. A wiring board for light-emitting element according toclaim 1, wherein the surface of said insulating substrate on the side ofthe light-emitting element mounting region has a total reflection factorof not lower than 70%.
 5. A wiring board for light-emitting elementaccording to claim 1, wherein said heat-conducting pole-like conductorhas a plane sectional area greater than that of the mounting area formounting the light-emitting element.
 6. A wiring board forlight-emitting element according to claim 1, wherein the boundaryportion between the end surface of said heat-conducting pole-likeconductor and the surface of said insulating substrate, and the vicinitythereof, are covered with a boundary protection layer formed by at leastone of those selected from the group consisting of a metal, ceramics anda resin.
 7. A wiring board for light-emitting element according to claim1, wherein the end surface of said heat-conducting pole-like conductoron the side of the mounting region and the peripheral edges thereof arecovered with a covering layer containing a metal or a resin.
 8. A wiringboard for light-emitting element according to claim 1, wherein the endsurface of said heat-conducting pole-like conductor on the opposite sideto the mounting region and the peripheral edges thereof are covered witha covering layer containing at least one of those selected from thegroup consisting of a metal, ceramics and a resin.
 9. A wiring board forlight-emitting element according to claim 1, wherein saidheat-conducting pole-like conductor has a thermal conductivity of notsmaller than 80 W/m·K.
 10. A wiring board for light-emitting elementaccording to claim 1, wherein said heat-conducting pole-like conductoris formed by using a metal material and a ceramic material.
 11. A wiringboard for light-emitting element according to claim 1, wherein saidheat-conducting pole-like conductor is incorporated in a portion ofelectric circuit.
 12. A wiring board for light-emitting elementaccording to claim 1, wherein said heat-conducting pole-like conductoris formed by using a plurality of layers having different coefficientsof thermal expansion or different thermal conductivities.
 13. A wiringboard for light-emitting element according to claim 1, wherein the sidesurface of said heat-conducting pole-like conductor is forming a tiltedsurface.
 14. A wiring board for light-emitting element according toclaim 1, wherein a step is formed on a side surface of saidheat-conducting pole-like conductor.
 15. A wiring board forlight-emitting element according to claim 14, wherein said insulatingsubstrate has a laminated-layer structure of a plurality of insulatinglayers, said step formed on the side surface of said heat-conductingpole-like conductor is positioned on the interface between theneighboring insulating layers, and a conducting layer is extending fromsaid step along the interface between said insulating layers.
 16. Awiring board for light-emitting element according to claim 15, whereinsaid conducting layer is extending from the edge of said step over alength of not smaller than 50 μm.
 17. A wiring board for light-emittingelement according to claim 1, wherein a frame is formed on saidinsulating substrate surrounding said light-emitting element mountingregion so as to protect the light-emitting element that is mounted. 18.A wiring board for light-emitting element according to claim 17, whereinthe inner wall surface of said frame has a total reflection factor ofnot smaller than 70%.
 19. A wiring board for light-emitting elementaccording to claim 17, wherein the inner wall surface of said frame hassuch a shape that the inner diameter at the upper end is greater thanthe inner diameter at the lower end, and is forming a curved surfacethat is swelling toward the side of said mounting region.
 20. A wiringboard for light-emitting element according to claim 17, wherein on theinner wall surface of said frame, there is provided a light-reflectinglayer comprising a gold-plated layer and a silver-plated layer formedthereon.
 21. A light-emitting device mounting a light-emitting elementon the mounting region of a wiring board for light-emitting element ofclaim 1.